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Immunodeficiency and other diseases

Many retroviruses infect blood cells, including cells of the immune system. For this reason, many of them induce immunodeficiency.
As we mentioned previously, the family Retroviridae is broad and complex and includes many important viruses of animals. It is distributed in a total of seven genera, of which the best known is that of Lentivirus, since a notorious representative of this genus is human immunodeficiency virus. In this genus, there are other viruses that cause immunodeficiency, including feline, simian, and bovine immunodeficiency, as well as others that produce slow diseases in sheep, goats and horses. Other important viruses are those that affect pets, such as feline leukaemia virus, and others that produce tumours in birds. Many retroviruses affect fish. The majority of these infections have in common that they develop slowly, in a matter of months or years.
Of all of them, we will focus primarily
on the two that affect cats: feline immunodeficiency and feline leukaemia. Feline leukaemia is caused by feline leukaemia virus, abbreviated as FeLV. It is known as the “friendly cat” disease because it is transmitted by saliva when cats groom each other, the mother licks the kittens, or when they share food or water containers. Because of this way of transmission, it is more frequent in young cats, in which the disease is also more severe, leading them to death in some months to a few years. As it is transmitted by these activities, the virus penetrates by the oral route.
If the cat’s immune response is effective, especially in older cats, they are able to fight the infection and eliminate the virus, either immediately (known as abortive infection), or after a brief transient viremia (known as regressive infection). But if the immune response is not effective, because they are young cats or they have a depressed immune system, viremia is prolonged and the virus entrenches in the bone marrow, infecting the blood cell precursors. The cat cannot eliminate the virus that remains latent. When the blood cell precursors divide, they carry the provirus, so infection destroys cells in the bone marrow, and spreads through blood circulation. The lack of immune cells allows the virus to reactivate and to be expressed.
When this occurs, it is known as progressive infection. In almost all cats, the progressive infection involves the presence of secondary infections, and the development of clinical signs and death in a short period of time. In a small percentage of cats, there can be atypical infections, with viral presence in sites different from those mentioned. Feline immunodeficiency is known as the “unfriendly cat” disease, because it is mainly transmitted by bites during fights between cats. For this reason, it is more frequent in male adults, older than 3 years of age, which are the ones that fight the most. As with the sadly famous human immunodeficiency virus that produces AIDS, feline immunodeficiency virus destroys the CD4+ T-lymphocytes that it infects.
This causes these cells, essential for the defence against pathogens, to decrease gradually. When there are not enough CD4+ lymphocytes, secondary infections develop, which produce a variety of clinical signs, and ultimately death. Despite the different mechanism by which FIV and FeLV produce disease, the clinical signs are very similar in infections by feline immunodeficiency virus and feline leukaemia virus, since in both cases they are partly a consequence of the immunodeficiency that is produced because the cells of the immune system are affected. Non-specific signs prevail, such as the lack of appetite, lack of energy, gingivitis, pallor of mucosa because of anaemia, weight loss and dishevelled coat, alterations in the eyes, etc. In many retroviral infections, the disease takes a long time to develop.
In many of them, when clinical signs appear, they are due to secondary infections, favoured by a lack of response to pathogenic microorganisms. Many animals are able to live with the retroviral infection during long periods, since the virus may remain dormant.

Retroviruses infect vertebrates, from fish to mammals.

Many of the diseases may go unnoticed because they develop slowly. Some others may be fulminant and lead to death in a few weeks. The latter are mostly produced by oncogenic retroviruses. Of all these, we have chosen feline leukaemia and feline immunodeficiency.

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Animal Viruses: Their Transmission and the Diseases They Produce

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